1. Field of the Invention
The present invention relates to a liquid-crystal display device comprising polarizing plates and retardation films, and in particular to a VA (vertically aligned)-mode liquid-crystal display device.
2. Related Art
It is known that a VA-mode liquid-crystal display device, comprising polarizers each disposed on and below a liquid-crystal cell with their absorption axes crossing vertically to each other, can achieve a wide viewing angle, or that is, can have improved display characteristics, by employing an optically biaxial retardation film disposed between each polarizer and the liquid-crystal cell (for example, Japanese Patent No. 3330574).
Heretofore, it is said that the retardation film to be used in the VA-mode liquid-crystal display device having the above-mentioned constitution must have retardation in plane (Re) of from 20 to 70 nm or so and retardation along thickness direction (Rth) of from 70 to 400 nm or so. Accordingly, it has been considered that, as the retardation film, a synthetic polymer film such as a polycarbonate film and a polysulfone film having a high retardation value (that is, having a large intrinsic birefringence) is advantageous.
On the other hand, a cellulose acylate film, especially a cellulose acetate film is characterized in that its intrinsic birefringence is small (that is, its retardation value is low), as compared with other polymer films. Accordingly, for use that requires optical isotropy, for example, for protective films for polarizer, cellulose acetate films are generally used.
EP-A 0911656A2 discloses a cellulose acetate film having a high retardation value and therefore applicable even to use that requires optical anisotropy, overturning a conventional general principle. In the document, an aromatic compound having at least two aromatic rings is added to a cellulose ester, thereby providing a cellulose ester film having a high retardation value. In general, it is known that cellulose triacetate is a hardly-stretchable polymer material and its birefringence is difficult to increase; however, by adding an additive thereto and stretching the film for alignment, the document has made it possible to increase the birefringence and to realize a high retardation value of the film. The film may serve also as a protective film for polarizer, and is therefore advantageous in that it may provide an expensive and thin-film liquid-crystal display device.
JPA No. 2005-134863 discloses an optical compensation sheet formed of a cellulose acylate film alone having Re of from 20 to 70 nm, Rth of from 70 to 400 nm and Re/Rth of from 0.2 to 0.4 and having a thickness of from 40 to 110 μm, to which is added a rod-shaped compound having at least two aromatic rings.